BUG: DataFrame.apply axis=1 for str ops with no axis argument (#39212)

Author: rhshadrach

doc/source/whatsnew/v1.3.0.rst

@@ -348,6 +348,7 @@ Reshaping
 - Bug in :func:`join` over :class:`MultiIndex` returned wrong result, when one of both indexes had only one level (:issue:`36909`)
 - :meth:`merge_asof` raises ``ValueError`` instead of cryptic ``TypeError`` in case of non-numerical merge columns (:issue:`29130`)
 - Bug in :meth:`DataFrame.join` not assigning values correctly when having :class:`MultiIndex` where at least one dimension is from dtype ``Categorical`` with non-alphabetically sorted categories (:issue:`38502`)
+- Bug in :meth:`DataFrame.apply` would give incorrect results when used with a string argument and ``axis=1`` when the axis argument was not supported and now raises a ``ValueError`` instead (:issue:`39211`)
 -
 
 Sparse

pandas/core/apply.py

@@ -193,6 +193,8 @@ def maybe_apply_str(self) -> Optional[FrameOrSeriesUnion]:
             sig = inspect.getfullargspec(func)
             if "axis" in sig.args:
                 self.kwds["axis"] = self.axis
+            elif self.axis != 0:
+                raise ValueError(f"Operation {f} does not support axis=1")
         return self.obj._try_aggregate_string_function(f, *self.args, **self.kwds)
 
     def maybe_apply_multiple(self) -> Optional[FrameOrSeriesUnion]:

pandas/tests/frame/apply/test_frame_apply.py

@@ -177,6 +177,16 @@ def test_apply_with_string_funcs(self, request, float_frame, func, args, kwds, h
         expected = getattr(float_frame, func)(*args, **kwds)
         tm.assert_series_equal(result, expected)
 
+    @pytest.mark.parametrize(
+        "how, args", [("pct_change", ()), ("nsmallest", (1, ["a", "b"])), ("tail", 1)]
+    )
+    def test_apply_str_axis_1_raises(self, how, args):
+        # GH 39211 - some ops don't support axis=1
+        df = DataFrame({"a": [1, 2], "b": [3, 4]})
+        msg = f"Operation {how} does not support axis=1"
+        with pytest.raises(ValueError, match=msg):
+            df.apply(how, axis=1, args=args)
+
     def test_apply_broadcast(self, float_frame, int_frame_const_col):
 
         # scalars